The NTSB's acting chairman Christopher Hart said while no cause for the crash had been determined, investigators found the "feathering" system - which lifts and rotates the tail to create drag - was activated before the craft reached the appropriate speed.

The system requires a two-step process to deploy. The co-pilot unlocked the system but Mr Hart said the second step occurred "without being commanded".

He said the investigation is months from being completed, and pilot error and mechanical failure are among many things being looked at.

The feathering is a feature unique to the craft to help it slow as it re-enters the atmosphere.

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After being unlocked, a lever rotates booms at the rear of the plane up to 90 degrees so they act as a rudder. After decelerating, the booms return to their normal position and the craft glides to Earth.

Mr Hart said the feathers activated at Mach 1.0, the speed of sound, but should not have deployed until it had at least reached a speed of Mach 1.4, or more than 1,000 mph.

(Image: Reuters)

Virgin Galactic - owned by billionaire Sir Richard Branson's Virgin Group and Aabar Investments PJS of Abu Dhabi - plans to fly passengers to altitudes more than 62 miles above Earth. The company sells seats on each prospective journey for 250,000 US dollars (£156,000).

Sir Richard had hoped to begin flights next year but said on Saturday that the project will not resume until the cause of the accident is determined and the problems fixed.

Physicist Stephen Hawking, singer Justin Bieber, comedian Russell Brand and actor Ashton Kutcher are among the celebrities who have already signed up for flights.

Virgin Galactic said it has taken deposits worth more than £50 million from about 700 people.

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